Unless provided with a fully equipped chemical laboratory, detection and identification of relatively dilute ambient concentrations of chemical vapors can represent a formidable task. Chemical vapor analyses are typically sensitive to only very specific vapors, generally producing a positive identification of a vapor through an irreversible chemical reaction. Automated apparatus to detect and identify chemical vapors have generally not been commercially available. The few portable chemical vapor detectors previously developed have required a relatively high concentration of a vapor in air or a relatively long exposure time to make a positive identification.
Chemical sensors have recently been developed that do not chemically react with a substance, but instead, physically change upon exposure to it. These chemical sensors typically include a polymer selected for its affinity to dissolve or absorb a group of related chemical vapors. As the chemical vapor dissolves or diffuses into the polymer, it causes a characteristic change in a parameter associated with the sensor. Perhaps the most fully developed of this type of chemical sensor is the surface acoustic wave (SAW) device. SAW devices suitable for use as chemical vapor sensors are readily available, for example, from a company that pioneered their development, Microsensor Systems, Inc., Fairfax, Virginia.
A method and apparatus for using a SAW device to detect a vapor is disclosed in U.S. Pat. No. 4,312,228. As described therein, the SAW device comprises a piezoelectric element having a surface coated with a material selected to interact with the chemical to be detected. Electrodes on the piezoelectric element are excited by a high frequency oscillator, producing a surface acoustic wave. Interaction of the chemical with the material coating the element alters one or more properties of the wave, and the electrodes on the piezoelectric element detect the altered wave, producing an electrical signal. The electrical signal is used to identify the chemical. While the material coating the surface of the element may be selected to chemically interact with the vapor in certain applications, to achieve reversible interaction, it is preferable to select a material for the coating that only physically interacts with the chemical.
The coating applied to a SAW device is typically a polymer selected for its characteristic interaction with the specific vapor or group of vapors to be detected. For example, fluoropolyol has been used as such a coating, since it absorbs a variety of chemical vapors. The absorption of a chemical vapor by the polymer coating of the SAW device increases the mass of the coating, and proportionally reduces the frequency of the surface acoustic wave propagating through the device. By monitoring the frequency of the surface acoustic wave, the absorption of a chemical substance into the polymer can be measured. Comparison of the measured absorption to the known characteristic solubility of the chemical vapor (or at least the class of chemical vapors) to which the sensor is exposed allows the determination of the ambient vapor concentration.
To qualitatively identify a specific chemical vapor, pattern recognition techniques are applied to an array of SAW devices that are each coated with different polymers. This method is described in a paper entitled, "Correlation of Surface Acoustic Wave Device Coating Responses With Solubility Properties and Chemical Structure," by D. S. Ballentine, Jr., S. L. Rose, J. W. Grate, and H. Wohltjen, published in Analytical Chemistry, Vol. 58, p. 3058, December 1986. As detailed therein, an evaluation was made using 12 SAW devices, coated with different polymer materials. The sensors were exposed to filtered air for one minute to establish a baseline response, followed by successive alternating exposures to a selected chemical vapor and the filtered air, each exposure lasting for approximately two minutes. The mass of the polymer coating increased exponentially as the chemical vapor was absorbed during each exposure, reaching an equilibrium level that varied as a function of the vapor concentration and of the affinity of each polymer coating to absorb that particular chemical vapor. During exposure to clean, filtered air, the chemical vapor exponentially desorbed from the polymer coating.
The normalized equilibrium concentration of a given chemical vapor in the polymer coating of each of the 12 SAW devices was evaluated using an eigenvector analysis technique. This analysis technique established sensor response patterns defining clusters for the response related to two classes of chemical vapors used in the test. It was found that four coatings, i.e., four different SAW devices, were sufficient to identify which of the two classes of chemical vapors were present, with almost 100% certainty.
There are two problems with the procedures used for identifying chemical substances in the above-described test. The prior method described relies upon determining the equilibrium concentration of a chemical vapor absorbed into the selected coating on a SAW device, providing only a single datum for the exposure of each device to a given chemical vapor. Furthermore, when the SAW device is exposed to very dilute concentrations of a chemical vapor, the time required for the chemical to absorb into the polymer coating to a level approaching its equilibrium concentration may be relatively long, for example, in excess of 15 minutes. In some applications, particularly those involving detection of harmful chemical vapors using a SAW device, it is critical that the chemical vapor be detected (and identified) before personnel in the area are adversely affected. The opposite problem arises where a chemical vapor is so highly concentrated that it saturates the polymer coating, making the SAW device insensitive to further exposure and use for an extended period of time.
Accordingly, it is an object of the present invention to rapidly detect a chemical substance present in relatively low concentrations and determine its ambient concentration. It is a further object to identify the chemical substance, or at least determine that it is from a known group of chemical substances. Yet a further object is to detect and/or identify a chemical substance based upon its predicted equilibrium concentration and its predicted time constant for diffusion into the coating of a sensor. Still a further object is to identify a chemical substance present in relatively high concentration, before it saturates the sensor. These and other objects and advantages of the present invention will be apparent from the attached drawings and the Description of the Preferred Embodiments that follows.